POSIX Regular Expressions

Tham khảo
The IEEE POSIX standard has three sets of compliance: BRE (Basic Regular Expressions),^[25] ERE (Extended Regular Expressions), and SRE (Simple Regular Expressions). SRE is deprecated,^[26] in favor of BRE, as both provide backward compatibility. The subsection below covering the character classes applies to both BRE and ERE.
BRE and ERE work together. ERE adds ?, +, and |, and it removes the need to escape the metacharacters ( ) and { }, which are required in BRE. Furthermore, as long as the POSIX standard syntax for regexes is adhered to, there can be, and often is, additional syntax to serve specific (yet POSIX compliant) applications. Although POSIX.2 leaves some implementation specifics undefined, BRE and ERE provide a "standard" which has since been adopted as the default syntax of many tools, where the choice of BRE or ERE modes is usually a supported option. For example, GNU grep has the following options: "grep -E" for ERE, and "grep -G" for BRE (the default), and "grep -P" for Perl regexes.

POSIX basic

In the POSIX standard, Basic Regular Syntax (BRE) requires that the metacharacters ( ) and { } be designated \(\) and \{\}, whereas Extended Regular Syntax (ERE) does not.

Metacharacter	Description
^	Matches the starting position within the string. In line-based tools, it matches the starting position of any line.
.	Matches any single character (many applications exclude newlines, and exactly which characters are considered newlines is flavor-, character-encoding-, and platform-specific, but it is safe to assume that the line feed character is included). Within POSIX bracket expressions, the dot character matches a literal dot. For example, a.c matches "abc", etc., but [a.c] matches only "a", ".", or "c".
[ ]	A bracket expression. Matches a single character that is contained within the brackets. For example, [abc] matches "a", "b", or "c". [a-z] specifies a range which matches any lowercase letter from "a" to "z". These forms can be mixed: [abcx-z] matches "a", "b", "c", "x", "y", or "z", as does [a-cx-z]. The - character is treated as a literal character if it is the last or the first (after the ^, if present) character within the brackets: [abc-], [-abc]. Note that backslash escapes are not allowed. The ] character can be included in a bracket expression if it is the first (after the ^) character: []abc].
[^ ]	Matches a single character that is not contained within the brackets. For example, [^abc] matches any character other than "a", "b", or "c". [^a-z] matches any single character that is not a lowercase letter from "a" to "z". Likewise, literal characters and ranges can be mixed.
$	Matches the ending position of the string or the position just before a string-ending newline. In line-based tools, it matches the ending position of any line.
( )	Defines a marked subexpression. The string matched within the parentheses can be recalled later (see the next entry, \n). A marked subexpression is also called a block or capturing group. BRE mode requires \( \).
\n	Matches what the nth marked subexpression matched, where n is a digit from 1 to 9. This construct is vaguely defined in the POSIX.2 standard. Some tools allow referencing more than nine capturing groups.
*	Matches the preceding element zero or more times. For example, ab*c matches "ac", "abc", "abbbc", etc. [xyz]* matches "", "x", "y", "z", "zx", "zyx", "xyzzy", and so on. (ab)* matches "", "ab", "abab", "ababab", and so on.
{m,n}	Matches the preceding element at least m and not more than n times. For example, a{3,5} matches only "aaa", "aaaa", and "aaaaa". This is not found in a few older instances of regexes. BRE mode requires \{m,n\}.

Examples:

• .at matches any three-character string ending with "at", including "hat", "cat", and "bat".
• [hc]at matches "hat" and "cat".
• [^b]at matches all strings matched by .at except "bat".
• [^hc]at matches all strings matched by .at other than "hat" and "cat".
• ^[hc]at matches "hat" and "cat", but only at the beginning of the string or line.
• [hc]at$ matches "hat" and "cat", but only at the end of the string or line.
• \[.\] matches any single character surrounded by "[" and "]" since the brackets are escaped, for example: "[a]" and "[b]".
• s.* matches s followed by zero or more characters, for example: "s" and "saw" and "seed".

POSIX extended

The meaning of metacharacters escaped with a backslash is reversed for some characters in the POSIX Extended Regular Expression (ERE) syntax. With this syntax, a backslash causes the metacharacter to be treated as a literal character. So, for example, \( \) is now ( ) and \{ \} is now { }. Additionally, support is removed for \n backreferences and the following metacharacters are added:

Metacharacter	Description
?	Matches the preceding element zero or one time. For example, ab?c matches only "ac" or "abc".
+	Matches the preceding element one or more times. For example, ab+c matches "abc", "abbc", "abbbc", and so on, but not "ac".
|	The choice (also known as alternation or set union) operator matches either the expression before or the expression after the operator. For example, abc|def matches "abc" or "def".

Examples:

• [hc]?at matches "hat", "cat", and "at".
• [hc]*at matches "hat", "cat", "hhat", "chat", "hcat", "cchchat", "at", and so on.
• [hc]+at matches "hat", "cat", "hhat", "chat", "hcat", "cchchat", and so on, but not "at".
• cat|dog matches "cat" or "dog".

POSIX Extended Regular Expressions can often be used with modern Unix utilities by including the command line flag -E.

Character classes

The character class is the most basic regex concept after a literal match. It makes one small sequence of characters match a larger set of characters. For example, [A-Z] could stand for the upper case alphabet, and \d could mean any digit. Character classes apply to both POSIX levels.
When specifying a range of characters, such as [a-Z] (i.e. lowercase a to upper-case z), the computer's locale settings determine the contents by the numeric ordering of the character encoding. They could store digits in that sequence, or the ordering could be abc…zABC…Z, or aAbBcC…zZ. So the POSIX standard defines a character class, which will be known by the regex processor installed. Those definitions are in the following table:

POSIX	Non-standard	Perl/Tcl	Vim	Java	ASCII	Description
	[:ascii:][27]			\p{ASCII}	[\x00-\x7F]	ASCII characters
[:alnum:]				\p{Alnum}	[A-Za-z0-9]	Alphanumeric characters
	[:word:][citation needed]	\w	\w	\w	[A-Za-z0-9_]	Alphanumeric characters plus "_"
		\W	\W	\W	[^A-Za-z0-9_]	Non-word characters
[:alpha:]			\a	\p{Alpha}	[A-Za-z]	Alphabetic characters
[:blank:]			\s	\p{Blank}	[ [[\t]]]	Space and tab
		\b	\< \>	\b	(?<=\W)(?=\w)|(?<=\w)(?=\W)	Word boundaries
				\B	(?<=\W)(?=\W)|(?<=\w)(?=\w)	Non-word boundaries
[:cntrl:]				\p{Cntrl}	[\x00-\x1F\x7F]	Control characters
[:digit:]		\d	\d	\p{Digit} or \d	[0-9]	Digits
		\D	\D	\D	[^0-9]	Non-digits
[:graph:]				\p{Graph}	[\x21-\x7E]	Visible characters
[:lower:]			\l	\p{Lower}	[a-z]	Lowercase letters
[:print:]			\p	\p{Print}	[\x20-\x7E]	Visible characters and the space character
[:punct:]				\p{Punct}	[][!"#$%&'()*+,./:;<=>?@\^_`{|}~-]	Punctuation characters
[:space:]		\s	\_s	\p{Space} or \s	[ \t\r\n\v\f]	Whitespace characters
		\S	\S	\S	[^ \t\r\n\v\f]	Non-whitespace characters
[:upper:]			\u	\p{Upper}	[A-Z]	Uppercase letters
[:xdigit:]			\x	\p{XDigit}	[A-Fa-f0-9]	Hexadecimal digits

POSIX character classes can only be used within bracket expressions. For example, [[:upper:]ab] matches the uppercase letters and lowercase "a" and "b".
An additional non-POSIX class understood by some tools is [:word:], which is usually defined as [:alnum:] plus underscore. This reflects the fact that in many programming languages these are the characters that may be used in identifiers. The editor Vim further distinguishes word and word-head classes (using the notation \w and \h) since in many programming languages the characters that can begin an identifier are not the same as those that can occur in other positions.
Note that what the POSIX regex standards call character classes are commonly referred to as POSIX character classes in other regex flavors which support them. With most other regex flavors, the term character class is used to describe what POSIX calls bracket expressions.